The present invention relates to vehicle diagnostic systems and, more particularly, the present invention relates to a vehicle diagnostic system which monitors one or more conditions of the vehicle""s brakes and tires and provides an indication of one or more conditions to the operator of the vehicle.
Operators of vehicles need to know the condition of the vehicle""s brakes and tires to ensure safe operation of a vehicle. To ensure safe operation of the vehicle, the vehicle""s brakes should not drag, the brakes should not fade, the temperature of the brakes should not be high enough to cause mechanical brake fade, the temperature of the tires should not be high enough to cause tire ply separation, the brakes should be properly adjusted, and the brake pads should have a sufficient amount of brake lining remaining.
Brake drag occurs when there is no gap between the brake pad and the brake drum when the brakes are released. When brakes are dragging, the brake pads are in continuous contact with the brake drum. Heat is generated by the frictional engagement between the brake pads and the brake drums as the vehicle wheels turn. The heat created by dragging brakes prematurely wears out the brake linings and the wheel bearings. In addition, if the heat is excessive enough for a prolonged period of time, the brake drums will expand or even crack and the tires may fail.
There are many types and causes of braking fade. Fade may result, for example, from a reduction in friction between linings and drums caused by exposure to water. Most typically, fade, resulting in a reduction of braking force, occurs when brakes drag on a long grade. If brakes are not properly adjusted, the drum will overheat and expand to an extent that pushrod travel is insufficient to fully actuate the brakes. This is one example of mechanical brake fade. Mechanical brake fade may also result from various mechanical defects in the brake system, such as scored brake drums. In contrast, heat fade occurs when the brake linings overheat and apply less frictional force when the linings are applied. It is desirable to provide a gradual and predictable warning of fade.
When the brake drums are excessively hot for a prolonged period of time, due to brake drag or prolonged application of the brakes, the brake drums will expand. Brake drums can reach temperatures of up to 800 to 1,000 degrees Fahrenheit with prolonged braking. Brake fade begins at approximately 300 degrees Fahrenheit and becomes significant at 500 degrees Fahrenheit. Brake drums increase in diameter by about 0.01xe2x80x3 per 100 degrees Fahrenheit. At 600 degrees Fahrenheit the brake drum diameter will have expanded by 0.055xe2x80x3 from the diameter at an average operating temperature of about 50 degrees Fahrenheit, causing the required air chamber push rod stroke to increase by approximately 0.40xe2x80x3, before the brake pads engage the brake drums. As the temperature of the system rises to a critical point, generally 300xc2x0 F., the brake lining itself becomes adversely affected and its frictional coefficient decreases, further reducing the braking system""s effective retardation force. This is known as brake lining fade. The problem is even more significant when lightweight centrifuge brake drums are used since they expand more readily than heavy cast drums. When the spring brakes of a parked vehicle with hot, expanded drums are applied, automatic slack adjusters may over-adjust and cause the brakes to drag.
Tires are heated up by being driven over hot pavement. Tires heat up faster when the load is excessive or the tire pressure is below recommended values. In addition, dragging brakes and prolonged braking produce significant heat that increases the temperature of the tires. As the tires heat up, the internal pressure of the tires increases and the tire rubber softens. The internal pressure of the tire increases by approximately 1 psi for each increase in temperature of 10 degrees Fahrenheit. Depending on the tire, tire ply to bead separation is possible at approximately 266 degrees Fahrenheit.
The brake pads of brakes that are under adjusted do not engage the brake pads to the brake drums with as much force as properly adjusted brakes. As a result, brakes that are under adjusted may not supply a sufficient amount of stopping power. Brakes that are over adjusted may drag causing unnecessary wear and could cause the brakes to overheat, resulting in brake lining fade. Again, the excessive heat could cause the brake drums to expand, resulting in mechanical fade or cracking of the drum. Brakes having linings that are worn beyond an acceptable level may not supply a sufficient amount of stopping power for safe operation of the vehicle.
It is known in the art to measure the angular rotation of an S-cam that brings the brake shoes and brake pads into frictional engagement with the brake drum to determine the adjustment of the brakes and wear of the brakes. However, to employ this method calculations must be performed to convert the angular rotation of the S-cam to an estimated travel distance at the brake shoe center. This calculation differs depending on the brake size and type, as well as the effective radius of the S-cam. S-cam type sensors that measure angular rotation must be configured to calculate the brake shoe travel of the particular brake type and size that it is monitoring.
An indication of imbalance between brakes on opposite sides of the vehicles is a difference in temperature of the brakes. Brake imbalance between brakes on opposite sides of the vehicle results in uneven braking force and possible vehicle instability due to brake steer. This condition may also cause uneven brake wear from side to side of the vehicle. An indication of brake imbalance between tandem axles is elevated temperatures of the brakes of one axle compared to the temperatures of the brakes of another axle. Another indication of brake imbalance between axles is a difference in brake lining thickness between the brakes of two different axles when the brakes of each axle were relined at the same time. Brake imbalance between axles results in uneven wear and reduced overall retardation force for the vehicle due to premature brake fade occurring on an overworked axle. The axle doing less work, due to poor adjustment, also applies less retardation force than is intended. Vehicle stability and controllability can be adversely affected if the brakes on one axle lock up prematurely. When drive axle brakes lock up, the tractor-trailer may jackknife. When trailer brakes lock up, the trailer may swing out of its traffic lane. When the steer axle locks up, a temporary loss of steering ability may occur.
What is needed is a vehicle diagnostic system which provides an indication to the operator of the vehicle in the vehicle cab, or to a technician from a data file, of one or more conditions of the brakes and tires of the vehicle.
A first exemplary embodiment of the present invention concerns a method and apparatus for indicating a condition of a vehicle brake assembly. The brake assembly has a brake shoe having an outer surface. A brake lining pad is connected to the outer surface of the brake shoe. A brake drum has an inner surface that surrounds the brake shoe and brake lining pad. The brake shoe and brake pad are movable from a disengaged position, where the brake pad is spaced apart from the brake drum, to an engaged position, where the brake pad is in forcible engagement with the brake drum. The apparatus includes a position sensor, a comparitor and an indicator. The sensor measures a gap between the brake lining pad and the inner surface of the brake drum in the disengaged position. The comparitor compares the measured gap to known values. The indicator is in communication with the comparitor. The indicator indicates brake conditions that are determined by comparing the measured gap to the known values in a way that is perceivable to a vehicle operator. The indicator alerts a vehicle operator to one or more brake and tire conditions.
In the first exemplary embodiment, the position sensor may measure the distance from the brake lining pad to the inner surface of the brake drum by measuring the relative linear motion of the brake shoe with respect to the position sensor when the brake shoe is moved between the disengaged and engaged positions.
In one application of the first exemplary embodiment, the indicator indicates brake drag when the measured gap is zero or less than the allowable brake drum run out.
In a second application of the first exemplary embodiment, the position sensor measures a distance between the brake shoe and the position sensor and the indicator provides an indication of the brake lining thickness remaining. In the second application of the first exemplary embodiment, the indicator may indicate the distance capable of being traveled safely before brake relining is required or the indicator may indicate that brake relining is required.
In a third application of the first exemplary embodiment, the indicator indicates that the brake assembly is out of adjustment.
In a fourth application of the first exemplary embodiment, the indicator indicates brake imbalance between first and second vehicle brake assemblies.
A second exemplary embodiment of the present invention concerns a method and apparatus for indicating a condition of a vehicle wheel assembly. The apparatus of this embodiment includes a sensor, first and second comparitors, a timer, and an indicator. The sensor is coupled to the wheel assembly for measuring a temperature of a wheel assembly component. The first comparitor compares the measured temperature of the component to a known threshold temperature. The timer measures a time period that the measured temperature exceeds the threshold temperature. The second comparitor compares the measured time period to a known soak time. The indicator is in communication with the first and second comparitors. The indicator indicates wheel assembly conditions that are determined by comparing the measured temperature and time periods to the threshold temperature and soak time.
In a first application of the second exemplary embodiment, the apparatus indicates brake fade. In this application, the sensor measures the temperature of a brake component. The indicator provides an indication that brake fade is possible when the measured temperature exceeds 300 degrees Fahrenheit for longer than the soak time. The indicator may indicate brake fade when the measured brake temperature exceeds 500 degrees Fahrenheit for longer than the soak time.
In a second application of the second exemplary embodiment, the indicator indicates whether tire ply separation is possible. In this application of the second embodiment, the sensor may measure the temperature of a tire directly. Alternatively, the tire temperature may be estimated by extrapolating the measured brake temperature.
In a third application of the second exemplary embodiment, the indicator indicates brake imbalance between first and second vehicle brake assemblies, when differences in temperature between the brake assemblies indicate brake imbalance. The temperature sensor may be a thermocouple embedded in the tire to read its temperature directly, a thermocouple mounted to the brake assembly, a frame mounted infrared sensor to read the temperature of the tire sidewall, or an infrared sensor mounted on the rim to measure air temperature inside the tire.
The position sensor may utilize radar for sensing the position of the brake shoe. Alternatively, the position sensor may be a linear variable differential transformer or potentiometric displacement transducer which is physically connected to the brake shoe. In one embodiment, a brake spider is operably connected to the brake shoe to facilitate movement of the shoe and pad between a disengaged position where the pad is spaced apart from the brake drum and an engaged position where the pad is in forceable engagement with the brake drum. A dust shield is connected to the brake spider. The dust shield has a first side that is oriented toward the brake shoe and a second side that is oriented away from the brake shoe. The dust shield includes an opening for a linkage in one embodiment.